Technical and economic analysis of fixed speed pump and variable speed pump feed water

1 Working principle and characteristics of fixed speed pump and speed control pump

When the pump is installed in a certain pipeline, the actual working state depends not only on the performance curve of the pump itself, but also on the characteristic curve of the entire pipeline. The intersection of these two curves determines the operating conditions of the pump in the piping system. In a device system with the same pipe characteristic curve, the operating points are different for different pumps. The working points of the fixed speed pump and the variable speed pump are very different, so that the flow, head and efficiency are different.

The fixed speed feed pump is generally centrifugal in the water supply system, and its working principle is to use the centrifugal force generated by the rotation to obtain energy for the fluid. It is driven by a certain power motor to drive the pump at a certain speed to obtain a certain pressure and kinetic energy of the fluid. The speed-regulating feed pump (for example, the YT62 type fluid coupling) has a special piece-hydraulic coupling. It is a hydraulic transmission device that is mounted between a prime mover (motor, etc.) and a feed water pump, and transmits power by the kinetic energy of the liquid. The working principle is as follows: there is one pump wheel at the end of the driving shaft, and there is a turbine at the end opposite to the driving shaft on the driven shaft, and a certain gap is maintained between the pump wheel and the turbine, and the two form a cavity type, For the cycle circle. When the prime mover drives the drive shaft to rotate, the working fluid gains energy in the pump wheel and is pumped out from the pump wheel at a certain speed, rushing toward the turbine, transferring energy to the turbine, thereby driving the passive shaft to rotate, realizing the transmission of power. .

The circulation of the working oil in the circulation circle can be maintained only when the pump wheel and the turbine have different rotational speeds (there is slip), that is, when there is a centrifugal force difference. The slip difference under rated operating conditions is ≤ 3%. As long as the amount of oil in the cycle circle is changed, that is, the oil filling amount is changed, the rotation speed and the transmission power of the driven shaft can be adjusted to achieve stepless speed regulation.

2 Technical and economic comparison of fixed speed pump and speed control pump

The eight-phase technical transformation project of Hebei Thermal Power Co., Ltd. is two 200 MW double-pumping heating units and four 410 t/h circulating fluidized bed boilers. Each 2 furnaces and one machine is one unit. It is planned to set the nameplate for each unit. Two speed control pumps with a power output of 440 t/h and a head of 1 400 m and a fixed speed pump. In the following, when the fixed speed pump (or speed governing pump) in the system is operated separately, two fixed speed pumps (or two speed regulating pumps) are operated in parallel, and one fixed speed pump and one speed regulating pump are operated in parallel.

2.1 Fixed speed pump (or speed control pump) is operated separately

Methods for changing the performance curve of the pump itself include shifting adjustment, bucket adjustment, and cavitation adjustment. The method of changing the pipeline characteristic curve mainly includes outlet throttling adjustment and inlet throttling adjustment, and the most commonly used adjustment mode is the outlet end throttling adjustment, which is a method of adjusting by adjusting the valve opening degree. Generally, the fixed speed pump is throttled at the outlet end, and the speed governing pump is adjusted for speed.

The start-up time of the boiler is long, and the whole pressure-lifting process of bringing the load of the boiler to 410 t/h is gradually achieved according to the requirements of the machine side, that is to say, if the speed-regulating pump is used in this process, the speed can be changed only by the speed. It can meet the start-up requirements of the furnace. It can be seen from the characteristic curve of the speed control pump that the corresponding head of the flow rate between 120 and 450 t/h can be raised from 800 m (or lower) to 1,400 m according to the stepwise temperature rise of the furnace, without adjusting the feed water regulating valve. When a fixed speed pump is used in this case, a long-term throttle feed water regulating valve is required to satisfy the required flow rate, so that a high energy head is consumed.

The flow rate is from 120 to 450 t/h, and the corresponding head is from 1 680 to 1 430 m. The excess energy head is consumed by the water supply regulating valve. See equation (1):

Where N—the power consumed by the motor, k W;
g———gravitational acceleration, m/s2;
Q———mass flow, kg/s;
H———Head, m;
Η—the efficiency of the pump, %.

It can be seen that power N and head H are directly proportional to other parameters. Since the speed governing pump can achieve the furnace start requirement only by changing the rotational speed without changing the pipeline characteristic curve, the energy saving effect is remarkable. For example, at 120 t/h, the speed control pump can save half the power of the fixed speed pump.

2.2 Parallel operation of the pump

The purpose of the parallel connection of the pumps is to increase the flow rate when the indenter is the same, and then draw the intersection point C of the common pipeline characteristic curve b and the parallel characteristic curve of the pump, that is, the working point when the parallel operation is performed. At this time, the flow rate is QC, and the head is HC. The parallel operation is characterized by: the heads are equal to each other, and the total flow rate is the sum of the flow rates of each pump (same performance pump), ie QC=2 QA. In this phase of the project, a 2-furnace 1 mechanism is used. When the second boiler is loaded with ignition, the second feed pump needs to be started. The following is a comparison of the parallel connection of two fixed speed pumps and the case of 1 fixed 1 and 2 adjusted.

2.2.1 Parallel operation of two fixed speed pumps in the water supply system

When the fixed speed pump is operated in parallel, the method of changing the characteristic curve of the pipeline is to adjust the opening degree of the boiler feed water regulating valve to achieve the purpose of adjusting the flow rate. The working point is that the head changes as the flow changes. The general design point is to design the flow rate, head and efficiency of the feed pump when the feedwater regulating valve is fully open. This is the economic working point, and the method of throttling adjustment is simple and convenient, but the overheating generated by the feedwater is too high. The regulating valve is consumed, resulting in energy loss, and the economy is worse at low loads. In particular, Hebei Thermal Power Co., Ltd. is a heating unit. The heat load is greatly affected by the industrial heat users and the heating heat load. The main feed water flow often changes. Adjusting the flow rate of the water supply regulating valve for a long time will definitely lead to the ratio. The pure condensing unit is more economically wasteful and causes serious damage to the flushing of the feed water regulating valve.

When the system is running at rated load (2 Q0), the head is the rated head H0 of the feed pump, and the flow rate is the sum of the rated flow rates of the two feed pumps. 2 Q0, the tube resistance curve at this time is b, that is, the feed water regulating valve is at full Open state; when the system is running at 0.75 rated load (1.5 Q0), the pipeline characteristic curve becomes b', that is, the feed water regulating valve is throttled, so that the working point of the parallel pump is from the original A. Change to A', the flow rate is 1.5 Q0, the head is H1, the working point of the corresponding pump changes from B to B', that is, the flow rate is 0.75 Q0, the head is H1, that is, the head is from the original H0. , added H1-H0.

2.2.2 Parallel operation of one speed control pump and one fixed speed pump

If each unit is 2 fixed speed pumps, add 1 speed control pump. In normal operation, the speed control pump is used to adjust the water supply flow required by the unit, so that the pressure of the water supply pipe is too high. This mode of operation requires that the outlet pressure of the variable speed pump be maintained at the rated outlet pressure of the fixed speed pump, and the flow rate varies between 30% and 100%, which requires simultaneous adjustment of the feed pump characteristic curve (by adjusting the feed pump speed) and Pipe resistance characteristic curve (by adjusting the feed water regulating valve opening degree). Let the operating point fall on the straight line segment with the head H.

In parallel operation, the operation is at point A under full load conditions, the flow rate is 2 Q0, the pressure head is H0, and both the fixed speed pump and the speed control pump are operated at point B. When the load is reduced to 1.5 Q0, the speed of the speed control pump is reduced, and the pump characteristic curve is changed to a2, so that the total lift curve of the feed pump group is a2'. At the same time, change the opening degree of the water supply regulating valve to make the pipe resistance characteristic curve to b'. At this time, the operating condition point is A0, the fixed speed pump is still running at point B (ie, the rated operating condition point of the pump), and the speed regulating pump is running at point B0, the flow rate is 0.5 Q0, and the pressure head is still H0. Thus, at a flow rate of 1.5 Q0, the energy consumption of the H1-H0 head difference is saved compared to the previously described pure fixed speed pump system.

If the pipe resistance characteristic curve is not adjusted at this time, the original pipe resistance characteristic curve can be extended downward to intersect with the characteristic curve of the original single fixed speed pump, and the load of the fixed speed pump will be found to be too high, and the speed governing pump can only carry less Load, which will lead to the problem that the fixed speed pump overload and the speed control pump load are too small and the heat, vibration and even vaporization are bad. Therefore, when there is a speed control pump, the method of adjusting the characteristic curve and the tube resistance characteristic curve of the feed pump should be adopted at the same time. Achieve the purpose of regulating the flow.

2.2.3 Two speed control pumps are operated in parallel

When the speed control pump is operated in parallel, the speed and flow rate of the speed control pump are changed to meet the requirements of the boiler. This avoids the high lift of the pump when the pump is running at a lower flow rate (below the rated flow rate) when the two fixed speed pumps are connected in parallel. The waste brought by it; overcomes the problem that when one boiler is fully loaded and the other boiler is ignited or with a small load, 1 fixed and 1 parallel operation causes the fixed speed pump to overload, and the speed control pump is too small to damage the pump. . The economic and safety of the unit is guaranteed.

3 Conclusion

In summary, the system in which the speed control pump and the fixed speed pump are connected in parallel saves the initial investment compared to the system that is all of the variable speed pump, and reduces the unit consumption of the water supply compared with the system of the fixed speed pump, especially as the heating unit. It is more reasonable to set up two speed-regulating pumps in the water supply system of the eight-phase technical transformation project of Hebei Thermal Power Co., Ltd., and it is more reasonable to use one fixed-speed pump as a backup.